Poly(ethylene terephthalate) (PET) polyesters are known to be useful for the manufacture of transparent bottles, jars, food trays, and other containers used for packaging beverages and food products. Although PET containers have good clarity, the gas barrier properties of PET polymers with regard to gases, such as, oxygen and carbon dioxide are generally insufficient. For example, oxygen permeability into PET bottles which contain beer, wine, and food products cause these products to spoil. Accordingly, PET containers are generally not used to manufacture containers or films for food products requiring long shelf life.
However, polyester films and bottles useful for packaging food products are known in the art. For example, a polyester blend possessing reduced carbon dioxide permeability and improved oxygen barrier properties is described in U.S. Pat. No. 4,578,437. The polyester blends of this patent comprise a mixture of from about 40 to about 95% by weight of one or more copolyesters having an inherent viscosity of from about 0.4 to about 1.0 and from about 5 to about 60% by weight of poly(ethylene isophthalate) (PEI).
Further, U.S. Pat. No. 5,183,623 describes a process for producing transparent and heat-resistant polyester films and containers by thermoforming techniques. The process forms a sheet from a molten polyester resin having ester units derived from 1,3-propanediol and terephthalic acid ester-forming derivatives thereof. The sheet is then quenched so as to form a solid resin sheet having low crystallinity. After aging, the sheet is shaped by thermoforming under specific conditions that result in a highly crystalline container.
Japanese Patent Application 08-104763 describes a polyester film made of poly(propylene terephthalate) (PPT). According to this Japanese application, improved film properties are obtained by limiting the number of carboxyl terminal groups in the PPT film to 40 Eq/t or smaller. Further, Japanese Patent Application 05-031789 (1993) describes a method for manufacturing a blown bottle made of polyester resin containing at least 80 mole % of repeat units comprising an ester unit prepared from terephthalic acid and 1,3-propanediol. This application teaches that to avoid problems associated with polyester blown bottles, i.e., heat shrinkage, a polyester having specific repeating units is used as the raw material which is drawn and blow molded under specific conditions.
U.S. Pat. No. 5,798,433 describes a process for producing PPT. The process comprises effecting the ester interchange or direct esterification and polycondensation reactions through the use of a unique catalyst system which comprises a tin compound employed in the polycondensation reaction.
In spite of the polyesters and processes discussed above, there remains a need for a polyester having improved oxygen and carbon dioxide barrier properties as well as improved crystallization rates. Such polyesters should be readily processable into containers having these improved barrier properties. In particular, these improved properties should be achieved without requiring the specific process steps or parameters described above. The present invention answers this need.